The present invention relates to a prism, widely usable with various precision optical instruments, in which provision is made to prevent detrimental internal reflection from any inoperative plane partially enclosing the prism.
Various types of prisms have been widely used with optical instruments to modify or deflect the path of light by reflection. Such prisms are solid blocks of transparent material having operational planes, such as light entry and light exit surfaces and internally reflective surfaces, and inoperative planes. The inoperative planes are not intended to serve as reflection surfaces, and hence there must be no light incident thereon.
But despite this intention, the inoperative planes, in actuality, are obliged to receive some incident rays of light because they must be near the path of light in order to make the prism smaller and lighter in weight. The rays of light incident upon the inoperative planes are internally reflected thereby and then travel as stray light, in some cases with repeating internal reflections, through the prism and the associated optical system. Such stray light is considered to be one of the factors by which ghosts and flares are produced on the image-forming plane of the optical system.
For the purpose of avoiding these detrimental internal reflections, it has been a usual practice to make the inoperative plane as an antireflection surface by rough-grinding it and then applying blacking thereto so as to absorb the rays of light incident thereupon. However, the recent improvement in the photosensitivity of photosensitive materials, photodetecting elements and the like renders even this measure insufficient.